I want to give users facility to download pdf files from my webpage. But if acrobat reader is already installed it will open the file and user have to save it manually. What I want to do is whether Acrobat reader is installed or not it must download the file rather than opening that file.

What should I do for this?

I don't think that there is a way to do this. If you have a link with a pdf file, the browser will automatically try to download it and open it.
The person can then choose to save the file after it has been opened.
I don't know why you want the file not to be opened.

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You can most probably do it with php or something alike. On SMF forums i've seen the force downloads of jpeg's which would normally just open in the current window.

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Better zip the file first and let them unzip and read it with their PDF reader. remember, there are other PDF reading tools available (Foxit for example).

I have already uploaded all the PDF files and i dont want to upload it again. I have heard that its possible. But dont know how.

In that case, let your user use Firefox by showing (better browsed with FF). Also, let them use the PDF Download extension (free from Mozilla). Once configured all PDF is automatically downloaded.

Since not all users will know what to do with just what I suggested above. And since you really wanted to do just what you say you wanted. Download PDF Download and modify portions to make it work the way you wanted and have users download that modified extension.

A hell lot of job but that's what web-developer means. (Ever seen a Chef relying on Waiter to cook the meal)

I already stated the basis of a way, maybe if you googled it you would of found your solution in less than 1 minute...

http://au3.php.net/header

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wish i could help you